Method and arrangement in a communication network

ABSTRACT

The present invention relates to the problem of establishing of security that arises within an ad hoc network 
     The problem is solved by using an optical device at a first device to read a public key that is encoded to a graphical string at a second device, which key is required for establishing security.

FIELD OF INVENTION

The present invention relates to the field of communication networks andmore specifically to an ad hoc communication network and a method forestablishing a security association in an ad hoc network.

DESCRIPTION OF RELATED ART

The fast growth of open networks with easy access has raised manysecurity problems. Several security solutions for public networks likethe Internet have appeared. Security is a problem in all kinds of opennetworks both wired and wireless. Information transmitted over the airis extremely vulnerable. Security solutions can be based on puresymmetric key techniques or can be a combination of symmetric andasymmetric, so-called public key techniques. Common solutions today arebuilt upon some type of so called Public Key Infrastructure (PKI). Apublic key infrastructure is a system used to distribute and checkpublic keys that can be used to authenticate users, exchange sessionkeys, sign information or encrypt information.

A symmetric key establishing scheme is built on that some a priorisecret is known by the involved parties in advance. In principle thereare two types of systems, key establishment between two parties sharinga common secret and key establishment by using a third party, a KeyDistribution Center (KDC). A typical requirement in any securityapplication is performing mutual authentication and key exchange. If thetwo involved parties, like in the first system, are pre-configured witha common shared secret this can be obtained by using a standardsymmetric key authentication and key exchange protocol.

A well-known example of the latter system is the Kerberos protocol. AKeberos system is shown in a schematic block diagram in FIG. 1. AKeberos system includes a central authentication server, the KDC 101 andseveral clients 102 and servers 103 whereof only one client 102 and oneserver 103 is depicted in FIG. 1. When a client 102 in the network wantsto exchange secure information with a server 103 in the network, aprotocol that involves communication with the KDC 101 according to thefollowing steps:

-   -   104. The client 102 sends a request including random number to        the KDC 101.    -   105. The KDC 101 replies to the client 102 with encrypted        session key    -   106. The client 102 sends the encrypted session key and        authenticator to the server 103.    -   107. The server 103 replies to the client 102 with an        authenticator. This step is an optional step.

The advantage with a system like the Kerberos system compared to mutualexchange is that each entity only needs to share one long lived key withthe KDC. There is no need to share keys with all parties in the network.The only entity that needs to store several long-lived keys is the KDC.

In a PKI system, two corresponding (also called asymmetric) keys areused in connection with protecting information. Information, which isencrypted with one of the two keys, can be decrypted only with the otherkey. In some PKI systems either of the two keys can be used to encryptand the other to decrypt. In other systems, one key must be used onlyfor encryption and the other for decryption. One important feature ofPKI systems is that it is computationally unfeasible to use knowledge ofone of the keys to deduce the other key. In a typical PKI system, eachof the systems possesses a set of two such keys. One of the keys ismaintained private while the other is freely published. If a senderencrypts a message with the recipient's public key, only the intendedrecipient can decrypt the message, since only the recipient is inpossession of the private key corresponding to the published public key.If the sender, before performing the above encryption, first encryptsthe message with the senders private key, the recipient, upon performingfirst a decryption, using the recipient's private key, then a decryptionon the result, using the sender's public key, is assured not only ofprivacy but of authentication since only the sender could have encrypteda message such that the sender's public key successfully decrypts it. Inone digital signature scheme, one-way hash is first applied to a messageand the hash of the message is encrypted with the sender's private key.

A PKI distributes one or several public keys and determine whether acertain public key can be trusted for certain usage or not. A piece ofdigitally signed information is often called a certificate. Certificatesare the basis upon which PKIs are built.

The degree of confidence that the recipient has in the source of amessage depends on the degree of the recipient's confidence that thesender's public key corresponds to a private key that was possessed onlyby the sender. In many current systems, a number of generally welltrusted certification authorities have been established to provide thisdegree of confidence.

A common certificate format is Standard X.509 (developed by theInternational Standards Organisation (ISO) and the Comité ConsultatifInternationale Telegraphique et Telephonique (CCITT)). Such acertificate may, e.g., include a public key, the name of subject whopossesses or is associated with the public key, an expiration date, allof which are digitally signed by a trusted party. The digital signaturemay be provided e.g., according to the digital signature standard (DSS)(National Institute of Standards and Technology (NIST)). Typically adigital signature involves applying a one-way hash and then encryptingwith the private key of, in this case, the certification authority. Suchdigital signature is provided using the private key of the trusted partywhich, in turn, is authenticated using the trusted party's certificatesigned by yet another trusted party, so that there may be a multi-levelhierarchy of trusted parties.

Another certificate format is Pretty Good Privacy (PGP) developed by P.Zimmermann and described in Internet Engineering Task Force (IETF) OpenPGP Specification. PGP provides a way to encrypt and decrypt, sign dataand exchange keys. Thus it is more than just a PKI. However, the mainidea with PGP is that no strict PKI is needed. Instead the PGP usersthemselves create and extend the PKI they need. This is done bycertifying other users public keys, i.e., signing trusted public keyswith their own secret key. In this way a “web of trust” is created. Aparticular key may have several different user IDs. Typically a user IDis an email address. If a revocation signature follows a key, the key isrevoked. A user certifies another users key by signing it with one ofthe keys of his own, which has signing capability. When signing anotherkey, different trust levels can be set, i.e., the amount of confidencethe signer has in the signed key and user ID.

Today, so-called ad hoc networks are used more and more frequently. Anad hoc network is established temporary for a special purpose. There isno fixed infrastructure; the nodes are the network. The nodes within thenetwork are often mobile and using radio links. An ad hoc network mightconstitute dynamic wide area connectivity in situations such as militaryoperations, rescue and recovery operations, and remote constructionsites. An ad hoc network might also constitute local area connectivityin situations such as temporary conference sites, home networks androbot networks. An ad hoc network might also constitute personal areanetworks in situations such as interconnected accessories, ad hocconference table and games. The nodes might consist of e.g. mobilephones, lap tops, television sets, washing machines In some situationslike in military operations or business conferences when thecommunication between the nodes comprises secrets, it is very importantthat a sender of a message can trust that the receiver really is theintended receiver.

In the previous examples, bindings between public keys and names orauthorisation are described. Several of these certificate solutionsexist in different systems. However, it is not yet described howdifferent certificates needed for different kinds of purposes areobtained. In the case of ordinary X.509 type of PKI with hierarchicalCertificate Authority (CA) structures, finding the right certificate isdone using some central on-line server or by direct transmission of thecertificate at connection set up. When using PGP either the desiredpublic key is stored locally on a machine or the device has to make aconnection to a central PGP server in order to find the desired pubickey. This works if it is possible for entities that need some type ofsecurity relation to have on-line connections to some particularservers. This is not the case for ad hoc networks. Ad hoc networks arecreated on the fly between entities that happen to be at the samephysical location.

Although all the security techniques described earlier are very powerfuland allow smooth and automatic security for many different use cases,they all have some problem when it comes to the special situation ofhuman faces in an ad hoc network.

Three different ad hoc scenarios will illustrate the shortcomings of therelated art described above regarding ad hoc security establishment.

In the first scenario several people gather together in a conferenceroom and would like to share some information. Everybody in theconference room has a communication unit such as a laptop or a PersonalData Assistant (PDA) with wireless access to all the other people in theroom. The people in the room have not been in contact with each otherpreviously. Now they would like to share some secret information using acertain application in their device. How can this be achieved?

In the second scenario, a person arrives at a new geographical locationand comes to some vendor machine offering him or her some type ofservice, e.g. like a ticket or some food. The person has a paying devicewith a wireless connection to the vendor machine. The company and theperson have no previous relation to each other. How can a persontransmit an electronic paying transaction (and thereby receive someproduct from the machine) to the vendor machine over the air interface?

Two different devices, e.g. a mouse and a Personal Computer (PC), fromtwo different vendors are connected to each other over a wireless link,in the third scenario. A person would like to “pair” these two devicesso that they can communicate securely over the wireless link How canthis be done in a user friendly and efficient way?

The symmetric key based key sharing mechanisms described above, alldemands that some secret information is shared between the devices thatwant to communicate. At least there must be a secure chain like inKerberos system that can be used to create a trust relation between twodevices. A secure chain is e.g. when A and B do not trust each other,but A and C trust each other, and B and C trust each other so A and Bcan get a trust relationship via C. This is often hard to achieve forthe first and second ad hoc scenario. Anyway, it would be verycumbersome to manually enter some secret information to all devices inthe first scenario. In the third scenario it would be possible to entersome secret symmetric information into the two devices that the personwould like to “pair” This is for example what is used in the securitysolution of the Bluetooth standard. However that means that if thedevice has no input channel, e.g. a mouse, a microphone etc., it must bepre configured with the secret information and this information must bekept secret Otherwise, anybody can make a pairing of the device.Furthermore, if the security level should be kept, the secret key ofsome certain device must be kept physically apart from the device. It ishard for humans to remember several Personal Identification Number (PIN)codes or to store them in a good and secure way.

A public key based system like the ones described above do not fit wellinto any of the scenarios described. If it should be possible to use aX.509 like certificate or a PGP key, a trusted party must sign thepublic key. In the first and second scenario it is not always assumedthat the parties share trusted public keys or have certificates signedby a third party that each party trust Also in the third scenario,certificates and public keys can not be used without some trust in thesignature of the certificate or a public key and since the devices cancome from any source it might be very hard to administrate distributionof trusted certificates to all possible devices.

Therefore, what is further needed is a way of making communicationswithin an ad hoc network more secure.

SUMMARY OF THE INVENTION

The present invention relates to the requirement of security in an adhoc network More particularly it relates to the problem of establishingof security that arises within an ad hoc network.

The problems discussed are:

The symmetric key based key sharing mechanisms described above, alldemands that some secret information is shared between the devices thatwant to communicate. This is often hard to achieve in ad hoc networks.

A public key based system like the ones described above do not fit wellinto ad hoc networks, since a trusted party must sign the public key. Itis unusual that the parties in an ad hoc network share trusted publickeys or have certificates signed by a third party that each party trust.

Accordingly, it is an object of the present invention to unravel theabove-mentioned problem.

The solution, according to the invention is to use an optical device toread a public key that is encoded to a graphical string, which key isrequired for establishing security.

An ad hoc communications network according to the invention includes afirst device and a second device. These devices are communicationdevices, which might be a laptop, a mobile phone, a printer, a vendormachine etc. The first device is equipped with an optical device. Thesecond device has a pair of keys, the key pair constituting a secret keyand a public key. The public key is hashed to a bit string which bitstring is encoded to a graphical string. The graphical string is visiblefor the user of the first device. The first device has a user, e.g. theowner of the first device that trusts the second device. The firstdevice wishes to authenticate the second device. The first device hasmeans for reading the graphical string by means of the optical deviceand means for authenticating the second device by means of the readstring including the public key. An ad hoc communications networkaccording to this first aspect of the invention is hereby characterisedby what are the features of claim 1.

A method for establishing a security relation between a first device anda second device within an ad hoc communications network according to asecond aspect of the invention, includes the steps of:

-   hashing the public key to a bit string;-   encoding the bit string to a graphical string;-   making the graphical string visible for the user of the first    device,-   the first device obtaining the graphical string by means of the    optical device, and-   the first device authenticating the second device by means of the    obtained graphical string.

A method according to this second aspect of the invention is herebycharacterised by what are the features of claim 6.

An advantage of the present invention is that it is possible to achievethe necessary security associations needed for distributing and sharinginformation among a group of users that happens to be at the samephysical location. There are a large amount of applications that fits into this scenario. Among those can be mentioned people from differentcompanies or organisations that gather in a conference room can sharedocuments with the meeting members.

Another advantage of the present invention is that the number ofmanually created trust relations between members in an ad hoccommunication network is decreased.

Yet another advantage of the present invention is that it makes itpossible “pairing” devices in a secure way also in the case of a devicelacking input channel

Yet another advantage of the present invention is that since the userphysically interacts with the other device to get the trusted key, it iseasier for the user to decide whether to trust a device or not.

Yet another advantage of the present invention is that due to thesimplicity of the solution, also people without much understanding ofthe rather complicated mathematics or principles of public keys, canmake secure connections with their devices.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 relates to Prior Art and is thus described above under“Description of related art”.

FIG. 1 shows a schematic block diagram of Keberos system.

FIG. 2 shows a schematic block diagram of an ad hoc communicationsnetwork according to the invention.

FIG. 3 shows a flowchart of the method according to the invention.

DESCRIPTION OF PREFFERED EMBODIMENTS

The ad hoc communications network according to the invention constitutese.g. a bluetooth network or a Wireless Local Area Network (WLAN). The adhoc network comprises devices constituting e.g. Personal Data AssistantsPDAs, lap tops, mice, mobile phones, vendor machines, paying devices,etc. each device comprising communication means. The devices areinterconnected via communication links.

FIG. 2 shows a possible scenario of an ad hoc communications network Naccording to the invention. The network N comprises a first device Awith wireless access to other devices within the network. The firstdevice A might be e.g. laptop. The first device A is connected to anoptical device O over a secured channel. The Optical device O readsinformation optically, i.e. code or text on paper or on an electronicslip, e.g. a LCD display. An example of such device is a so-called CPen™.

The first device A also has a person that uses it, a user UA, e.g. theowner of the device.

The user UA wishes to communicate with a second device B within thenetwork N. The second device B has a wireless access to other deviceswithin the network and it might be e.g. a laptop, a vendor machine, aservice device etc. The second device B might also have a user UB ormight not, as in the case of constituting a vendor machine or a servicedevice. The second device B has one or several secret key-public keypairs. The public key might be contained in a certificate signed by athird party. The public key or certificate that an arbitrary devicewould like to use to authenticate itself towards the second device B and/or exchange keys, is hashed, using a cryptographic strongone-way-function (see A. J. Menzes, P. C. van Orschot and S. A.Vanstone, Handbook of Applied Cryptography, CRC Press, 1997) to a largeenough (to provide enough cryptographic strength) bit string. The bitstring is mapped by a one-to-one code to a suitable graphical string S,that is readable for the optical device O. The graphical string S insome way visible for the user AU and the first device A, it might beprinted on a card carried by the owner or user UB of the second deviceB, or it might be displayed on a slip, possibly electronic, physicallyattached to the second device B.

The user UA requires to create a security association between his ownfirst device A and the second device B. The user AU, who trusts thegraphical string S, reads the graphical string S with the optical deviceO. The user UA trusts the graphical string e.g. if it is printed on acard that he got from user UB who he knows or trusts by any other means,or by recognising a trustworthy company trademark of a vendor machine onwhich the slip, displaying the graphical string, is attached. Tosimplify for a user to trust a slip displaying a string it can beconstructed so that it is easy for a user to see that nobody hasmanipulated the slip or that there is some electronic protection of theslip that disables the second device B if somebody manipulates the slip.

The read graphical string is transmitted from the optical device O tothe first device A in a secure way, if they are in different entities.

The first device A gets the graphical string. If later the devicereceives a public key or a certificate containing the public key thatcan be hashed to the string S, that public key or certificate will betreated as trusted.

The first device A contacts the second device B and performs thesecurity protocol. The security protocol used for authentication andshared key generation can be of any standard type like the TransportLayer Security (TLS) handshake protocol or the Internet Key ExchangeProtocol (IKE).

The first device A authenticates the second device B using the publickey that S is a graphical string of If the second device B is able toproof that it holds a secret key corresponding to the public key that Sis a graphical string of, the second device B is trusted by the firstdevice A.

It is possible for the user UA to decide for how long and to what extenda public key corresponding to the graphical string should be trusted. Inmany situations this trust relation might last for a very short timeperiod.

In another example, both the first and the second devices A and B have arespective optical device and a respective key pair encoded into arespective graphical string being visible. So if the connection betweenthe first device A and the second device B is a mutual trustedconnection, The first and the second device A and B exchange secretsession keys using trusted public keys.

In an embodiment of the present invention the second device Bconstitutes a service device which has a network address. The servicedevice C might be a printer, a camera, a projector, a pay machine etc.The first device A which wishes to connect to the service devicerequires the network address. According to the present invention thegraphical string S is mapped to the network address of the servicedevice B. When the first device A reads the graphical string S by meansof the optical device O, it obtains the public key, but also the networkaddress of the service device B.

FIG. 3 shows a flowchart of establishing a security relation between afirst device and a second device within an ad hoc communicationsnetwork, according to the invention in a general mode.

The first device having an optical device and the second device having apair of keys constituting a secret key and a public key.

The first device has a user that trusts the second device.

The method comprises the following steps:

-   301. The public key is hashed to a bit string.-   302. The bit string is encoded to a graphical string.-   303. The graphical string is made visible for the user of the first    device.-   304. The first device obtains the graphical string by reading the    visible optical string by means of the optical device.-   305. The first device authenticates the second device by means of    the obtained graphical string.

1. An ad hoc communications network comprising: a first device having an optical device; and a second device having a pair of keys, the pair of keys comprising a secret key and a public key, the first device having a user that trusts the second device wherein the public key is hashed to a bit string, wherein the bit string is encoded to a graphical string, the graphical string being visible for the user of the first device, the first device having means for obtaining the graphical string by means of the optical device, and the first device having means for authenticating the second device by means of the obtained string.
 2. The ad hoc communications network according to claim 1, wherein the first device, after receiving the public key from the second device, trusts the public key if it can be hashed to the graphical string.
 3. The ad hoc communications network according to claim 1, wherein the second device comprises a service device having a network address and the graphical string is mapped to the network address.
 4. The ad hoc communications network according to claim 3, wherein the first device has means for obtaining the network address, by means of the optical device.
 5. The ad hoc communications network according to claim 4, wherein the first device has means for connecting to the service device by means of the obtained network address.
 6. Method for establishing a security relation between a first device and a second device within an ad hoc communications network, the first device having an optical device, the second device hiving a pair of keys comprising a secret key and a public key, the first device having a user that trusts the second device, the method comprising the steps of: hashing the public key to a bit string; encoding the bit string to a graphical string; making the graphical string visible for the user of the first device; the first device obtaining the graphical string by means of the optical device; and the first device authenticating the second device by means of the obtained graphical string.
 7. The method according to claim 6, wherein the first device, after receiving a public key from the second device, trusts the public key if it can be hashed to the string.
 8. The method according to claim 6, wherein the second device comprises a service device having a network address, the method comprising the further step of mapping the graphical string to the network address.
 9. The method according to claim 8, comprising the further step to be taken by the first device: obtaining the network address by means of the optical device.
 10. The method according to claim 9, comprising the further step to be taken by the first device: connecting to the service device by means of the obtained network address. 